Atherosclerosis, a common form of arteriosclerosis, results from the deposition of fatty substances, primarily cholesterol, and subsequent fibrosis in the inner layer (intima) of an artery, resulting in plaque deposition on the inner surface of the arterial wall and degenerative changes within it. The ubiquitous arterial fatty plaque is the earliest lesion of atherosclerosis and is a grossly flat, lipid-rich atheroma consisting of macrophages (white blood cells) and smooth muscle fibers. The fibrous plaque of the various forms of advanced atherosclerosis has increased intimal smooth muscle cells surrounded by a connective tissue matrix and variable amounts of intracellular and extracellular lipid. At the luminal surface of the artery, a dense fibrous cap of smooth muscle or connective tissue usually covers this plaque or lesion. Beneath the fibrous cap, the lesions are highly cellular consisting of macrophages, other leukocytes and smooth muscle cells. As the lesions increase in size, they reduce the diameter of the arteries and impede blood circulation resulting in coronary heart disease, myocardial infarction (MI) and other serious complications.
Many therapies have been considered for the treatment of atherosclerosis, including surgery and medical treatment. One potential therapy is percutaneous transluminal angioplasty (balloon angioplasty). More than 400,000 such procedures are performed each year in the United States. In balloon angioplasty, a catheter equipped with an inflatable balloon is threaded intravascularly to the site of the atherosclerotic narrowing of the vessel. Inflation of the balloon compresses the plaque enlarging the vessel.
While such angioplasty has gained wider acceptance, it suffers from two major problems, i.e., abrupt closure and restenosis. Abrupt closure refers to the acute occlusion of a vessel immediately after or within the initial hours following a dilation procedure. Abrupt closure occurs in approximately one in twenty cases and frequently results in myocardial infarction and death if blood flow is not restored in a timely manner.
As many as 50% of the patients who are treated by balloon angioplasty require a repeat procedure within six months to correct a re-narrowing of the artery. Restenosis refers to such re-narrowing of an artery after an initially successful angioplasty. Restenosis of the blood vessel is thought to be due to injury to the endothelial cells of the blood vessel during angioplasty, or during inflation of the balloon catheter. During healing of the blood vessel after surgery, smooth muscle cells proliferate faster than endothelial cells resulting in a narrowing of the lumen of the blood vessel and starting the atherosclerotic process anew. In recent years, smooth muscle cell proliferation has been recognized as a major clinical problem limiting the long-term efficacy of coronary angioplasty.
In an effort to prevent restenosis of the treated blood vessel, the search for agents that can reduce or prevent excessive proliferation of smooth muscle cells have been the object of much research. (The occurrence and effects of smooth muscle cell proliferation after these types of surgery have been reviewed, for example, in Ip, et al., (June 1990) J. Am. College of Cardiology 15:1667-1687, and Faxon, et al. (1987) Am. J. of Cardiology 60: 5B-9B.). Such compounds have found little if any practical success. There therefore exists a need to identify and successfully administer compounds that inhibit smooth muscle cell proliferation.
An alternative to angioplasty is the placement of endovascular stents in the occluded blood vessel. Placement of a stent at such a site, should mechanically block abrupt closure and delay restenosis (Harrison's Principles of Internal Medicine, 14th Edition, 1998). Of the various procedures used to overcome restenosis, stents have proven to be the most effective. Stents are metal scaffolds that are positioned in the diseased vessel segment to create a normal vessel lumen. Placement of the stent in the affected arterial segment prevents recoil and subsequent closing of the artery. By maintaining a larger lumen than that created using balloon angioplasty alone, stents reduce restenosis by as much as 30%. Despite their success, stents have not eliminated restenosis entirely. (Suryapranata et al. 1998. Randomized comparison of coronary stenting with balloon angioplasty in selected patients with acute myocardial infarction. Circulation 97:2502-2502).
Unfortunately, the use of such stents are limited by direct (subacute thrombosis) or indirect (bleeding, peripheral vascular complications) complications. After stent implantation the patients are threatened with stent thrombosis until the struts of the stent are covered by endothelium. Thus, an aggressive therapy using anticoagulation and/or antiplatelet agents is necessary during this period of time. While these therapies are able to decrease the rate of stent thrombosis, they are the main source of indirect complications.
In addition to coronary artery occlusion, narrowing of the arteries can occur in other vessels. Examples include the aortoiliac, infrainguinal, distal profunda femoris, distal popliteal, tibial, subclavian and mesenteric arteries. The prevalence of peripheral artery atherosclerosis disease (PAD) depends on the particular anatomic site affected as well as the criteria used for diagnosis of the occlusion. Rates of PAD appear to vary with age, with an increasing incidence of PAD in older individuals. Data from the National Hospital Discharge Survey estimate that every year, 55,000 men and 44,000 women had a first-listed diagnosis of chronic PAD and 60,000 men and 50,000 women had a first-listed diagnosis of acute PAD. Ninety-one percent of the acute PAD cases involved the lower extremity. The prevalence of comorbid coronary artery disease (CAD) in patients with PAD can exceed 50%. In addition, there is an increased prevalence of cerebrovascular disease among patients with PAD.
PAD can be treated using percutaneous transluminal balloon angioplasty (PTA). The use of stents in conjunction with PTA decreases the incidence of restenosis. However, the post-operative results obtained with medical devices such as stents do not match the results obtained using standard operative revascularization procedures, i.e., those using a venous or prosthetic bypass material. (Principles of Surgery, Schwartz et al. eds., Chapter 20, Arterial Disease, 7th Edition, McGraw-Hill Health Professions Division, New York 1999).
Preferably, PAD is treated using bypass procedures where the blocked section of the artery is bypassed using a graft. (Principles of Surgery, Schwartz et al. eds., Chapter 20, Arterial Disease, 7th Edition, McGraw-Hill Health Professions Division, New York 1999). The graft can consist of an autologous venous segment such as the saphenous vein or a synthetic graft such as one made of polyester, polytetrafluoroethylene (PTFE), or expanded polytetrafluoroethylene (ePTFE). Restenosis and thrombosis, however, remain significant problems even with the use of bypass grafts. For example, the patency of infrainguinal bypass procedures at 3 years using an ePTFE bypass graft is 54% for a femoral-popliteal bypass and only 12% for a femoral-tibial bypass.
Consequently, there is a significant need to improve the performance of both stents and synthetic bypass grafts in order to further reduce the morbidity and mortality of CAD and PAD.
With stents, the approach has been to coat the stents with various anti-thrombotic or anti-restenotic agents in order to reduce thrombosis and restenosis. For example, impregnating stents with radioactive material appears to inhibit restenosis by inhibiting migration and proliferation of myofibroblasts. (U.S. Pat. Nos. 5,059,166, 5,199,939 and 5,302,168). Irradiation of the treated vessel can pose safety problems for the physician and the patient. In addition, irradiation does not permit uniform treatment of the affected vessel.
Numerous attempts to develop stents with a local drug-distribution function have been made, most of which are variances of the so called stent graft, a metal stent covered with polymer envelope, containing a medicament. It would be of benefit to coat a stent with a compound capable of diminishing or eliminating restenosis.
Unlike the unwanted smooth muscle cell proliferation seen in restenosis, cellular proliferation is a normal ongoing process in all living organisms and is one that involves numerous factors and signals that are delicately balanced to maintain regular cellular cycles.
When normal cellular proliferation is disturbed or somehow disrupted, the results can be inconsequential or they can be the manifestation of an array of biological disorders. Disruption of proliferation could be due to a myriad of factors such as the absence or overabundance of various signaling chemicals or presence of altered environments. Some disorders characterized by abnormal cellular proliferation include cancer, abnormal development of embryos, improper formation of the corpus luteum, difficulty in wound healing as well as malfunctioning of inflammatory and immune responses.
Cancer is characterized by abnormal cellular proliferation. Cancer cells exhibit a number of properties that make them dangerous to the host, often including an ability to invade other tissues and to induce capillary ingrowth, which assures that the proliferating cancer cells have an adequate supply of blood. One of the defining features of cancer cells is that they respond abnormally to control mechanisms that regulate the division of normal cells and continue to divide in a relatively uncontrolled fashion until they kill the host.
It is clear that aberrant cellular proliferation plays a major role in the formation and progression of a cancer. If this abnormal or undesirable proliferative activity could be repressed, inhibited, or eliminated, then the tumor, although present, would not grow. In the disease state, prevention of abnormal or undesirable cellular proliferation could slow or abate the progression of cancer. Additionally, compounds that could induce apoptosis of abnormally proliferating cells would be especially beneficial for complete removal or elimination of malignant cells, helping to reduce relapses. Therapies directed at control of the cellular proliferative processes could lead to the abrogation or mitigation of such malignancies.
Pulmonary hypertension is caused largely by an increase in pulmonary vascular resistance and is classified clinically as either primary or secondary. Secondary pulmonary hypertension, the more common form, is generally a result of (1) chronic obstructive or interstitial lung disease; (2) recurrent pulmonary emboli; (3) liver disease; or (4) antecedent heart disease. Primary pulmonary hypertension is diagnosed only after all known causes of increased pulmonary pressure are excluded.
At the moment there is no successful cure for pulmonary hypertension. Administration of vasodilatating drugs has not proved to be useful in patients suffering from pulmonary hypertension. The prognosis is poor, with a median survival time of about 3 years.
Pulmonary fibrosis can occur in response to known stresses such as asbestos or silica but most is idiopathic. There is a spectrum of idiopathic fibrosis but most kinds are fatal in 3-5 years. At present, there is no effective therapy for most cases.
What is needed therefore is a composition and method which can inhibit abnormal or undesirable cellular proliferation, especially the growth of smooth muscle cells after angioplasty, stent placement, pulmonary hypertension, pulmonary fibrosis or the proliferation of malignant cells. The composition should be able to overcome the activity of endogenous growth factors in premetastatic tumors and inhibit smooth muscle cell proliferation during restenosis. Finally, the composition and method for inhibiting cellular proliferation should preferably be non-toxic and produce few side effects.
Heparin is a glycosaminoglycan that was first described by McLean in 1916 and has been used clinically as an anticoagulant for more than 50 years [McLean, Circulation 19, 75-78 (1959)]. Members of the glycosaminoglycan family include hyaluronan, heparan sulfate, dermatan sulfate, and chondroitin sulfate. Beyond its well-recognized anticoagulant activity, heparin has other activities. The antimetastatic activity of heparin has been known for some time (see, for example, Drago, J. R. et al., Anticancer Res., 4(3), 171-2, 1984).
Unfortunately, native and currently described modified heparins are extremely anticoagulant. Their anticoagulant properties are such that doses effective in the treatment of malignancies and anti-proliferative disorders are not attainable. It has therefore been suggested that altering the chemical structure of heparin might decrease the anticoagulant properties of heparin while maintaining its other important biological activities, such as its antimetastatic activity (Barzu et al., J. Med. Chem, 1993, 36, pg. 3546-3555).
Low molecular weight heparins have shown promise in reducing anticoagulation while maintaining their antimetastatic activity. For example, when compared to unmodified heparin, 2-O-desulfated and 3-O-desulfated heparins had reduced anticoagulant activities, but preserved their angiostatic, anti-tumor and anti-metastatic properties (Masayuki et al., U.S. Pat. No. 5,795,875 (1997); Lapierre et al., Glycobiology 6, 355-366 (1996)]. Nevertheless, the use of currently available heparins and heparin derivitives for the treatment of abnormal cellular proliferative disorders is not practical due to their marked anticoagulant and antithrombotic activities.
O-acylated heparins have been described. These molecules have very low anticoagulative effects in vitro, yet retain activity against HIV-1 and 2 induced cytopathicity (Barzu et al., J. Med. Chem, 1993, 36, pg. 3546-3555).
A chemically modified heparin that can be used to treat and/or prevent abnormal cellular proliferative disorders is needed. Such a compound should have minimal anticoagulant properties while maintaining antiproliferative properties. The anticoagulative properties of the compound must not limit its use in the clinical setting.